Green Fabrication of Stackable Laser-Induced Graphene Micro-Supercapacitors under Ambient Conditions: Toward the Design of Truly Sustainable Technological Platforms

RESEARCH ARTICLE www.advmattechnol.de Green Fabrication of Stackable Laser-Induced Graphene Micro-Supercapacitors under Ambient Conditions: Toward the Design of Truly Sustainable Technological Platforms Sara L. Silvestre,* Maria Morais, Raquel R. A. Soares, Zachary T. Johnson, Eric Benson, Elisabeth Ainsley, Veronica Pham, Jonathan C. Claussen, Carmen L. Gomes, Rodrigo Martins, Elvira Fortunato, Luis Pereira, and João Coelho* Extensive research into green technologies is driven by the worldwide push for eco-friendly materials and energy solutions. The focus is on synergies that prioritize sustainability and environmental benefits. This study explores the potential of abundant, non-toxic, and sustainable resources such as paper, lignin-enriched paper, and cork for producing laser-induced graphene (LIG) supercapacitor electrodes with improved capacitance. A single-step methodology using a CO2laser system is developed for fabricating these electrodes under ambient conditions, providing an environmentally friendly alternative to conventional carbon sources. The resulting green micro-supercapacitors (MSCs) achieve impressive areal capacitance (≈7–10 mF cm−2) and power and energy densities (≈4𝝁Wcm -2 and ≈0.77 μWh cm−2 at 0.01 mA cm−2). Stability tests conducted over 5000 charge–discharge cycles demonstrate a capacitance retention of ≈80–85%, highlighting the device durability. These LIG-based devices offer versatility, allowing voltage output adjustment through stacked and sandwich MSCs configurations (parallel or series), suitable for various large-scale applications. This study demonstrates that it is possible to create high-quality energy storage devices based on biodegradable materials. This development can lead to progress in renewable energy and off-grid technology, as well as a reduction in electronic waste. S. L. Silvestre, M. Morais, R. Martins, E. Fortunato, L. Pereira, J. Coelho CENIMAT i3N Department of Materials Science School of Science and Technology NOVA University Lisbon and CEMOP/UNINOVA Caparica Portugal E-mail: [email protected];[email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.202400261 © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/admt.202400261 1. Introduction Sustainability is now the driving force for a greener and more technologically advanced future, in line with the global goals set out by the United Nations 2030 Agenda.[1]Therefore, to facilitate the transition to a sustainable world and to meet the increasing demand to reduce global dependence on fossil fuels and electronic waste (e-waste), it has become crucial to explore the potential of abundant and renewable materials.[2–4]Among the natural materials available, cellulose, and lignocellulosic by-products are noteworthy, as they are biodegradable and hold promise for the development of green electronics.[2,5] In addition to their environmental benefits, these materials offer several advantages over traditional polymer substrates, including cost effectiveness, ease of handling, and improved customization for a wide range of electronic applications, including disposable wearable R. R. A. Soares, Z. T. Johnson, J. C. Claussen, C. L. Gomes Department of Mechanical Engineering Iowa State University Ames, IA 50011, USA E. Benson, E. Ainsley, V. Pham School of Art +Design Graphic Design Program University of Illinois Urbana-Champaign Champaign, IL 61820, USA J. Coelho Dpto. Física de la Materia Condensada Universidad de Sevilla Avda. Reina Mercedes S/N, Sevilla 41012, Spain J. Coelho Instituto de Ciencia de Materiales de Sevilla (Universidad de Sevilla-CSIC) Avda. Americo Vespucio 49, Sevilla 41092, Spain Adv. Mater. Technol. 2024,9, 2400261 2400261 (1 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH www.advancedsciencenews.com www.advmattechnol.de electronics, paper-based circuits, and packaging solutions.[6] However, not all cellulose-based electronics are straightforward in terms of complexity.[7]For example, paper-based electronics typically involve complex processes such as inkjet-based deposition, lithography, and spray coating, among others.[8]While these approaches enable the creation of innovative electronic functionalities on sustainable substrates, they still raise concerns in terms of complexity, limited design flexibility, and time required to manufacture specific devices.[9]The application of laser technology has proven to be an impressive solution to overcome these challenges, offering a simpler, faster, and more controlled methodology.[10]This cost-effective approach not only enables the large-scale conversion of natural, abundant, and renewable resources into green laser-induced graphene (gLIG) structures with ease, but it also offers environmental advantages over conventional LIG fabricated on commercial materials such as polyimide.[10–12]In addition, gLIG fulfills essential and desired electrode requirements – high electrical conductivity and surface area. This property is primarily due to the inherent nature of the substrates, which results in a highly porous structure with a substantial surface area.[11]Furthermore, the high defect density in the structure of gLIG can enhance its electroactivity and catalytic potential for various electrochemical applications.[13]Therefore, the promising properties of gLIG have led to its use in a variety of fields, including the fabrication of biosensors,[14–16]supercapacitors,[17–22]humidity and temperature sensors,[23]mechanical sensors,[24,25]among others. When designing sustainable platforms, there is a need for a stable and continuous power supply for various functions, including sensing, data communication, and signal conditioning.[7]Therefore, researchers have explored micro-supercapacitors as an effective and reliable energy storage solution.[26]MSCs have the potential to meet these requirements and withstand adverse conditions, especially where conventional power sources are limited. They offer several advantages over other storage devices, such as highpower density, fast charging and discharging, and long cycle life.[27,28]Various gLIG-MSCs have been developed on different substrates such as paper, leaves, lignin, wood, cork, and others, demonstrating different ranges of specific capacitance and overall electrochemical performance.[11,13]Although LIG fabrication is a straightforward method, it often results in materials with different morphologies and properties, making device and substrate comparison a cumbersome process.[11]In this work, gLIG is fabricated using a CO2laser under ambient conditions and is assembled into sandwich-shaped micro-supercapacitors. First, the laser writing process is optimized on different substrates (paper, lignin-enriched paper, and cork) to obtain LIG with high electrical conductivity and the highest quality. This optimization is controlled through Raman spectroscopy by mapping and studying the characteristic graphitic peaks. This research is characterized by the exploration sustainable and abundant substrates while also investigating the contribution of lignin substrates to improve LIG production. Following this, MSCs are assembled to understand the influence of LIG properties derived from the different substrates on the electrochemical performance of the devices. This study also demonstrates a novel ability to interconnect individual MSC units in both series and parallel configurations. This distinctive adaptability significantly increases the overall capacity of the system, enabling tailor-made applications. In addition, an environmentally conscious approach to energy has been introduced with the development of green LIG MSCs. These assembled devices have also demonstrated their effectiveness by successfully powering electronic devices, such as a watch integrated with a humidity and temperature sensor. This demonstration highlights the high potential and practical utility of the devices fabricated, positioning our work as a pioneering contribution in the field of LIG-based energy storage platforms. 2. Results and Discussion 2.1. LIG Fabrication and Characterization Laser conversion for the production of gLIG using various renewable materials has been extensively studied, using bio-based carbon sources and substrates such as wood, leaves, cork, husks, and cellulose.[11]This work aims to investigate and compare the conversion of three cellulosic substrates: paper, lignin-enriched paper, and cork. In particular, lignin-enriched paper is a variant produced by environmentally friendly industries that emphasize the incorporation of lignin content – often considered a by-product and waste – into recycled substrates or related byproducts. Higher lignin content in substrates has been shown to be beneficial in producing higher quality LIG with more desirable Raman profiles, characterized by lower defect density in the graphitic structures formed.[11]This ensures a better carbon hybridization yield during laser conversion, which directly improves the conductive properties of the LIG, as discussed below.[29] Recognizing the importance of selecting the most appropriate precursor for each target application, this research thoroughly investigates the tunning of LIG production from these substrates, thus providing the flexibility to tune the LIG electrode properties and architecture. Due to the highly flammable nature of the materials used, the substrates were impregnated with a borax fire retardant solution prior to laser exposure, followed by a drying process. This measure is intended to protect the substrate from thermal degradation and prevent undesirable effects such as substrate ablation and the formation of fragile LIG structures. Similar issues have previously been observed in studies with other cellulosic substrates.[14,21]Furthermore, the borax pre-treatment is a critical step in the process, mainly because these natural substrates are converted into LIG in a non-inert atmosphere.[11]Some reports show that the use of boric acid in substrate pretreatment can lead to a boron doping effect during electrode fabrication.[30–32]As far as we are concerned, sodium tetraborate was not used as a doping agent, but rather as a fire retardant for the formation of LIG on these highly sensitive substrates. It cannot be excluded that some boron doping takes place on the prepared samples. However, in our case it is not possible to compare the LIG properties on a treated substrate with those on untreated samples. Therefore, the extent and significance of boron on the prepared LIG cannot be properly assessed and further characterization was not pursued. After impregnating the substrates with a flame-retardant to fine-tune the LIG properties, different laser conditions such as the laser defocus, power, and speed were investigated, Adv. Mater. Technol. 2024,9, 2400261 2400261 (2 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2024, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admt.202400261 by Universidad De Sevilla, Wiley Online Library on [27/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmattechnol.de Figure 1. a) Matrix of sheet resistance values for different laser settings (in percentage) and b) SEM (Scanning electron microscopy) images of the topview and cross-section (green highlighted) of the borax-paper (BP), borax-lignin-enriched paper (BLEP), and borax-cork (BC) laser-induced graphene fabricated using the optimal laser settings (identified on the bottom right corner) corresponding to the lowest sheet resistance. demonstrating the importance of precise laser settings to achieve distinct LIG properties, which have a direct impact on the electrochemical performance of LIG-based MSCs.[33,34]By first optimizing the defocus, a foundation was laid for the subsequent adjustments to power and speed, as the study of defocus conditions helped to initiate the optimization process and minimize potential issues of overheating and substrate damage. For example, adjusting the defocus distance from the focal point leads to changes in the energy distribution over the area undergoing LIG conversion, which affects the substrate carbonization rate and LIG formation, homogeneity, thickness, porosity, conductivity, crystallinity, among other factors.[11,35,36]Therefore, a laser beam position of 2 mm below the focal point was set as the optimum parameter for all substrates. This setting results in the lowest sheet resistance, as shown in Figure S1 (Supporting Information). This property has previously been highlighted as one of the most important aspects to be improved when fine-tuning of LIG electrode fabrication to meet the electrical conductivity criteria for desired electronic applications. The defocus distance was consistently maintained throughout the subsequent work, proving to be an effective strategy to avoid simultaneous changes in multiple parameters. Continuing the evaluation of the LIG from different substrates, the analysis was extended to assess their morphology and surface topology. Figure 1ashows a comprehensive analysis of the correlation between the variations in power and speed applied during laser irradiation and the resulting changes in LIG sheet resistance values for all substrates. This matrix shows not only the sheet resistance values but also discloses the influence of the laser parameters on the electrical properties of each substrate. The matrix values are expressed as percentages, considering the maximum power and grating speed of the CO2laser system are 75 W and 3.05 m s−¹, respectively. In addition to the matrix of sheet resistance values, Figure 1b complements the analysis by showing top view and crosssectional SEM images of the three different LIG substrates produced at optimal laser settings. The scale has been adjusted to match the dimensions of the samples, with the aim of achieving higher resolution in the cross-sectional images. These images provide valuable morphological information on the surface Adv. Mater. Technol. 2024,9, 2400261 2400261 (3 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2024, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admt.202400261 by Universidad De Sevilla, Wiley Online Library on [27/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmattechnol.de topography, layering, and thickness of the LIG. This morphological information can be used to correlate the LIG microstructure with its conductivity. The observed reduction in sheet resistance values, traced along the diagonal of each matrix, is consistent with existing literature.[14,17,21]For the paper and lignin-enriched paper substrates, the optimal conditions were obtained at a laser power of 4% and a speed of 13% (3 W @ 0.40 m s−1) resulting in sheet resistances of 55.4 ±2.8 ohm sq−1(for paper) and 45.5 ±2.3 ohm sq−1(for lignin-enriched paper), respectively. Similarly, for the cork substrate, the optimal LIG electrode formation was achieved at a laser power of 4% and a speed of 6% (3 W @ 0.18 m s−1), yielding a sheet resistance of 10.6 ±0.5 ohm sq−1. Notably, a higher degree of graphitization of the substrates with higher lignin content was observed. A comparison of paper and lignin-enriched paper with cork reveals that cork exhibits greater tolerance and thermal resistance to laser irradiation, thereby preserving its structural integrity even at lower processing speeds. This is due to the fact that paper substrates are more susceptible to rapid heating and combustion during laser irradiation due to their higher thermal conductivity compared to cork. As previously reported, the thermal conductivity values for cellulose paper and cork are ≈0.076 and 0.045 W m−1K−1, respectively.[37,38] This fact highlights the importance of the substrate composition and its inherent characteristics before laser conversion, which are determinants for the successful formation of conductive LIG. It is noteworthy that the sheet resistance values of cork’s are significantly lower than those of paper and lignin-enriched paper while maintaining the integrity of the underlying substrate. Despite flame-retardant pre-treatment, the susceptibility of sensitive substrates to ablation is a common phenomenon.[14,21]Nevertheless, this issue can be effectively addressed by applying higher processing speeds, and in this study, a speed two times higher for paper and lignin-enriched paper than the cork substrate was employed. This approach minimized the potential ablative effect on these substrates while still enabling the production of LIG with good electrical properties.[11,28]Additionally, sheet resistance can also be influenced by the morphology of the LIG obtained. As evidenced by existing literature,[10,39,40]it is well-established that the morphological structure of LIG can directly affect its electrical conductivity. Therefore, Figure 1b presents surface and crosssectional views of the LIG samples produced from the different substrates, which yielded the lowest and optimal sheet resistance values. The images show a uniformly interconnected and porous LIG network for all substrates, which plays a pivotal role in the efficient charge transport within its framework. When examining the relationship between LIG’s thickness and its sheet resistance, it can be observed that thicker LIG derived from cork exhibits a distinctive advantage in terms of electrical conductivity compared to LIG derived from paper and lignin-enriched paper. This characteristic may appear contradictory because thicker films usually exhibit lower conductivity when compared to thinner layers. Thinner layers offer a higher density of conductive pathways within the LIG matrix, often leading to improved conductivity.[10] Nevertheless, when thicker LIG possesses a well-connected network of conductive pathways, it can facilitate the formation of an increased abundance of graphene layers. Consequently, this offers a multitude of routes for charge carriers to move within its structure, ultimately promoting higher electrical conductivity. To further understand the properties and potential applications of the fabricated LIG, the samples were studied by Raman spectroscopy. As depicted in Figure 2, the obtained spectra suggest the successful formation of LIG on paper, lignin-enriched paper, and cork substrates. In Figure 2a, Raman mapping provides insights into the structural variations of LIG produced on these natural substrates along their surface. The I2D/IGand ID/ IGratios were evaluated by tracking the characteristic graphitic peaks measured at different locations on the samples and by assessing their intercorrelation. In Figure 2b, the peaks show the formation of LIG in all samples with characteristic Raman peaks identified. These include peaks D (≈1350 cm−¹), G (≈1580 cm−¹), and 2D (≈2700 cm−¹).[11,13]The color variations observed in each mapping result from increased or decreased peak intensity, which provides valuable information about the material’s composition and quality. Lower ID/IGratios indicate the presence of higher-quality graphene domains with a lower density of defects in the formed graphitic structures.[29]This characteristic is prominently observed on lignin-enriched paper and cork substrates, where the D peak intensity is notably reduced. In contrast, when using a pure paper substrate, a prevalence of the D peak over the G peak is observed, indicative of a higher density of defects, and consequently, lower-quality LIG. The phenomenon of reduced D peak intensity has been reported for substrates with higher lignin content, resulting in an increased degree of graphitization. This consistent with the decreasing trend in sheet resistance, which follows the order: BP-LIG (≈55.4 ohm sq−1), BLEP-LIG (≈45.5 ohm sq−1), and BC-LIG (≈10.6 ohm sq−1).[41]Upon correlation with SEM images, borax-cork LIG appears to be thicker than paper-based LIG and exhibits a more uniform and interconnected lattice structure. Consequently, cork-based LIG exhibits a lower sheet resistance than paper-based LIG. Conversely, the presence of the 2D peak indicates the formation of graphitic material, and the I2D/IGratios are often used as criteria to identify the number of graphene layers.[11,42]Therefore, the higher intensity of 2D peaks identified in paper-based LIG compared to cork-LIG, along with higher I2D/IGratios, is mainly attributed to fewer graphene layers in the material. To support the results above, X-ray photoelectron spectroscopy (XPS) characterization was conducted, which showed the distinct presence of graphene structures.[11,17,21]This confirmation is derived from an analysis of the high-resolution spectrum of the C 1s in all samples (Figure S2, Supporting Information).[11] 2.2. MSCs Fabrication and Stack Assembly The fabrication process for the single-device MSCs is visually illustrated in Figure 3. This process involved three distinct steps: a) electrode fabrication via laser-inducing technique, subsequent b) deposition of the electrolyte, and the final c) assembly of the components into functional MSCs. The solid PVA/H2SO4elec- trolyte was carefully deposited onto the LIG electrodes through a drop-casting technique (Figure 3b). Next, a vacuum process was employed for 10 min. This additional step was designed to promote increased electrolyte penetration into the LIG structure, enhancing the contact area between the electrolyte and electrode. A Adv. Mater. Technol. 2024,9, 2400261 2400261 (4 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2024, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admt.202400261 by Universidad De Sevilla, Wiley Online Library on [27/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmattechnol.de Figure 2. Raman a) mapping and b) spectra of borax-paper, borax-lignin-enriched paper, and borax-cork LIG. paper separator was employed to prevent device short-circuiting problems and retain the electrolyte within the desired area. The adoption of the sandwich configuration over the interdigitated one was intended to fulfill the energy-related targets set forth for systems that require enhanced energy and power density, efficient electrode-electrolyte interaction, and integration in compact formats. For multiple connected LIG electrodes, a single-step patterning approach of multiple electrodes was initially considered. However, this approach has drawbacks, including limited independence from each individually fabricated MSC and potential short-circuit issues. An alternative strategy emerged: patterning distinct electrodes separately to create single MSCs and then assembling them one by one. This method allows for the integration of the desired number of MSCs, achieved by gently connecting each unit through their silver pads, as seen in Figure 4. This strategy enhances the control over the final energy storage device as it enables individual replacement of MSC, the addition or removal of units for specific application needs, adaptability across diverse applications, and reusability. Integration of Figure 3. Schematic illustration of a single LIG-MSC fabrication from a) electrode preparation, b) electrolyte deposition, and c) device assembly. Adv. Mater. Technol. 2024,9, 2400261 2400261 (5 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2024, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admt.202400261 by Universidad De Sevilla, Wiley Online Library on [27/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmattechnol.de Figure 4. Illustration of single MSC alongside stacked MSCs in both series and parallel arrangements. stacked MSCs offers significant benefits to the energy storage system, including increased energy and power density, enhanced total capacitance, improved energy efficiency, and customization flexibility.[43,44]Consequently, this strategy was subjected to exploration and is depicted in Figure 4, which presents single MSC and stacked MSCs arrangements arranged in series and parallel configurations, thereby demonstrating the reliability of the assemblies for compact energy storage solutions. The subsequent section presents a comparative analysis of the electrochemical performance of various configurations and the number of employed MSC units. It highlights the stacked configuration’s capability to meet diverse energy storage requirements. 2.3. Electrochemical Performance of Sandwich Supercapacitors A comprehensive assessment of the sustainable MSCs was conducted to explore their energy storage and delivery capabilities. This evaluation involved the examination of cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) analyses performed across all devices. In Figure 5a,b, illustrates the presence of leaf-like shaped CV curves for MSCs derived from BP and BLEP. In contrast the BC MSC (Figure 5c) exhibits a quasirectangular shape, which is indicative of the typical electrical double layer (EDL) capacitive behavior.[45] Figure 5a-c illustrates the impact of varying scan rates on the diffusion of ions within the electrode. At lower scan rates, ions have more time to diffuse and penetrate the deeper pores of the electrode, leading to enhanced areal capacitance. As the scan rate increases from 5 to 100 mV s−1, the distinct leaf-like shaped CV curves become more pronounced due to constrained ion diffusion. This is typically observed when the available time for ion diffusion is reduced at higher scan rates.[27]The BC MSCs benefit from the inherent substrate characteristics, including greater porosity, increased LIG thickness, and a larger surface area.[18] These attributes contribute to maintaining quasi-rectangular CV curves even at higher scan rates, which highlights the cork electrode’s capability to withstand more challenging conditions. This resilience is crucial for applications requiring rapid charge and discharge cycles. The GCD graphs demonstrate near-symmetrical triangular curves across all devices, in accordance with the typical EDL behavior observed in the CV analysis. The observed asymmetry in the LIG-derived MSCs’ charge and discharge times can be attributed to ion kinetics within the porous electrode structure. This is influenced by electrochemical reactions occurring at the electrode-electrolyte interface.[17]Factors such as porosity, thickness, and the presence of surface functional groups within the electrode matrix contribute to variations in ion transport rates.[11,13]Despite the observed variations, all devices exhibit relatively similar behavior, suggesting that the overall electrochemical performance remains consistent across all devices. Figure 6apresents a summary of the calculated areal capacitance values of all devices as the current density increases from 0.01 to 0.2 mA cm−2, as extracted from the curves. At a current density of 0.01 mA cm−2, the highest capacitance of 7.34 mF cm−2was achieved for BP MSC, 9.51 mF cm−2for BLEP MSC, and 8.65 mF cm−2for BC MSC. Table 1compares the performances of gLIG and polyimidebased LIG MSCs, in addition to the values obtained from this study. The inclusion criteria considered a range of factors, including electrode designs, laser types, substrate type, optimized laser conditions, and those with the highest relevance and closest similarity to the presented study. The capacitances of the best-performing devices in this work, when compared with those reported in the literature, suggest reliability in the obtained results. This contributes to a growing body of knowledge around gLIG as a promising alternative for energy storage devices. In this study, the sandwich configuration was selected over the interdigitated, primarily due to the objective of prioritizing a larger effective surface area for the electrode-electrolyte interface, following reports that demonstrated improved capacitance.[18] While certain applications may prioritize high packing density, often favored by interdigitated designs, others may prioritize a greater effective surface area, favored by a sandwich Adv. Mater. Technol. 2024,9, 2400261 2400261 (6 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2024, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admt.202400261 by Universidad De Sevilla, Wiley Online Library on [27/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmattechnol.de Figure 5. Cyclic voltammograms of a) BP, b) BLEP, and c) BC based LIG MSCs; respectively, and corresponding galvanostatic charge and discharge curves on (d–f). configuration. Nevertheless, exploring the most suitable configuration requires a careful balance of multiple factors, including electrode thickness, electrolyte used, ion diffusion paths, application-specific needs, among others.[27,45]These considerations are essential for optimizing the overall performance and efficiency of the supercapacitor system. Furthermore, this study aimed to assess the suitability and versatility of renewable materials in the practical development of MSC. All devices showed relatively similar electrochemical performances (Figure 6). However, under higher current densities, BC MSC consistently demonstrated superior performance. This resilient behavior suggests a notable capacity to sustain performance under increased current densities, making it particularly intriguing and promising candidate for applications that demand stable electrochemical performance. The Ragone plots in Figure 6b confirm the high-energy and high-power density achieved for all devices. At a current density of 0.01 mA cm−2, the highest energy density (EA) values were 0.73, 0.85, and 0.77 μWh cm−2for BP, BLEP, and BC MSCs, respectively. At the same current value, the power density (PA)was able to reach 4 μWcm −2for all substrates. Figure 6. a) Areal capacitance (CA) from GCD analysis and b) ragone plot of all devices. Adv. Mater. Technol. 2024,9, 2400261 2400261 (7 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2024, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admt.202400261 by Universidad De Sevilla, Wiley Online Library on [27/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmattechnol.de Table 1. Comparison of LIG-based MSCs capacitances from literature, using direct laser writing method. Electrode Design Laser Electrolyte Capacitance References LIG/Borax-paper LIG/Borax-lignin-enriched paper LIG/Borax-cork Sandwich CO2PVA-H2SO4(BP) 7.34 mF cm−2 (BLEP) 9.51 mF cm−2 (BC) 8.65 mF cm−2 @0.01mAcm −1 This work LIG/Natural cork Sandwich UV PVA-H2SO44.13 mF cm−2 @0.05mAcm −2 [18] LIG/Kraft lignin Sandwich CO2PVA-H2SO42.51 mF cm−2 @0.01mAcm −2 [19] LIG/Cork Interdigitated Fiber PVA-H2SO41.43 mF cm−2 @0.1mAcm −2 [17] LIG/Borax-paper Interdigitated CO2PVA-H2SO44.6 mF cm−2 @ 0.015 mA cm−2 [21] LIG/Polyimide Interdigitated CO2PVA-H2SO44mFcm −2 @0.2mAcm −2 [46] LIG/Polyimide Sandwich CO2PVA-H2SO49mFcm −2 @0.02mAcm −2 [47] A stability evaluation was conducted through 5000 cycles of charge–discharge (Figure 7) at a current density of 0.08 mA cm−2. During the cyclability test of all devices, it was observed that the paper-based MSCs demonstrated slightly better capacitance retention (≈85% of initial capacitance) compared to the cork MSCs (≈80% of initial capacitance). This capacitance loss of capacitance upon cycling is not ideal, but it is not uncommon. Some residual species resulting from the LIG processing are likely to exhibit irreversible reactions or poorly bound particles that degrade the capacitance during the first measurement cycles. Other authors also report that during the charge/discharge cycles of these devices, the observed loss of capacitance can be attributed to factors such as corrosion of electrode and substrate materials, decomposition of electrolyte water, and formation of gases in electrode pores, among others. Nevertheless, it is observed that after 3000 Figure 7. Comparison of the capacitance retention versus cycle number obtained from GCD curves of BP-LIG, BLEP-LIG, and BC-LIG MSC at a current density of 0.08 mA cm−2. cycles, the MSCs capacitance tends to stabilize over time as expected, which is consistent with the literature.[17,21] While the observed performance provides valuable insight into the potential applications of these devices, it is also crucial to also consider material stability and electrode-electrolyte interaction under demanding conditions over extended cycles. Compared to cork, paper substrates are typically thinner and more fragile, which may limit their suitability for prolonged use, particularly when using harsh electrolytes such as standard PVA-H2SO4, leading to substrate degradation over time. Nevertheless, BC MSCs demonstrate robustness and longevity, making them more suitable for energy storage applications in real-world scenarios, despite slightly lower capacitance retention compared to other materials. Further electrochemical characterization, namely Electrochemical impedance spectroscopy (EIC), can be found in the Supporting Information. 2.4. Modular Assembly Characterization – Stacked and Sandwiched MSCs To supply energy for practical applications, a single MSC is insufficient to power a system, so MSCs should be integrated into series or parallel configurations. This is essential to overcome the inherent limitations of the capacity of a single device. Figure 8a–c, GCD curves are presented, comparing a single device with two and three-stacked devices integrated into both series and parallel configurations. These experiments were conducted at a current density of 0.05 mA cm−2. The GCD curves reveal the expected behavior of EDL capacitors, characterized by a quasi-triangular shape and a slight iR drop across all devices.[48]Nonetheless, these curves show distinct performances among the different configurations and quantities of stacked devices. Compared to a single device, the stacked series MSC demonstrates a cumulative voltage, featuring a 2×higher working voltage window (1.6 V) for two devices and a 3×higher working voltage window (2.4 V) for three devices, respectively. In contrast, Adv. Mater. Technol. 2024,9, 2400261 2400261 (8 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2024, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admt.202400261 by Universidad De Sevilla, Wiley Online Library on [27/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.advmattechnol.de Figure 8. Electrochemical performances of single and stacked LIG-MSCs in series and parallel circuits. GCD curves comparing a single device to stacked series and parallel devices at a current density of 0.05 mA cm−2in a) BP, b) BLEP, and c) BC based LIG MSCs. the stacked parallel configuration exhibits a 2×longer discharge time for two devices and a 3×longer discharge time for three devices at the same current density. Consequently, this results in an enhanced overall capacitance achieved by adding MSCs. The extended discharge times emphasize the potential for increased energy storage capabilities, making parallel configurations favorable for applications requiring prolonged power delivery at lower voltage.[47]Moreover, the basic operational characteristics of both series and parallel connections are demonstrated by all MSCs, reinforcing the adaptability and feasibility of these configurations. Corresponding CV curves for stacked series and parallel MSCs can be found in Figure S4 (Supporting Information). The versatility of integrating MSCs allows for tailored setups to meet specific application requirements, optimizing their performance for a diverse range of practical uses in electronic systems. As an example, and to validate their practicability, three BC MSCs were stacked in series, highly extending the voltage window from 0.8 to 2.4 V. A practical demonstration (Figure 9) shows that the assembled MSCs were able to successfully power both a multicolor light-emitting diode (LED) with a typical forward voltage in the range of 2–3.2 V and a humidity/temperature sensor with a forward voltage requirement of 1.5 V. This demonstration not only proved the capability and simplicity of stacking and connecting multiple MSCs according to specific voltage requirements but also emphasized their potential across different applications. Videos demonstrating both examples can be found in the Supporting Information. The intermittent red, green, and blue (RGB) LED was powered for ≈30 s, while the humidity and temperature sensor were powered for roughly 3 min. The modular setup allows scalability, enabling the stacking of additional micro-supercapacitor units for increased energy storage and prolonged device operation. Moreover, these configurations have the potential to facilitate the development of energyefficient, self-sustainable systems, particularly in low-power, lowresource, and remote sensing applications. 3. Conclusion In summary, micro-supercapacitors were fabricated using green LIG electrodes were successfully fabricated on abundant, renewable, and non-toxic substrates, such as paper, lignin-enriched paper, and cork. The laser parameters were systematically optimized for each substrate, and a flame-retardant was incorporated to ensure efficient electrode production while preventing combustion and substrate ablation. Cork exhibited the lowest sheet resistance (≈10.6 ohm sq−1), followed by lignin-enriched paper (≈45.5 ohm sq−1) and paper (≈55.4 ohm sq−1). This can be attributed to inherent material characteristics such as thermal resistance and lignin content. The sustainable and stackable sandwich microsupercapacitors demonstrated capacitance values of 7–10 mF cm−2, which is consistent with prior research. The integration of these green LIG-based energy storage devices in various configurations offers potential for multifunctional platforms in renewable energy and off-grid technology. The study found out that cork is the preferred substrate material due to its natural Adv. Mater. Technol. 2024,9, 2400261 2400261 (9 of 12) © 2024 The Authors. Advanced Materials Technologies published by Wiley-VCH GmbH 2365709x, 2024, 16, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admt.202400261 by Universidad De Sevilla, Wiley Online Library on [27/09/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License